[GHC] #15557: Reduce type families in equations' RHS when testing equation compatibility

[GHC]

#15557: Reduce type families in equations' RHS when testing equation compatibility
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        Reporter:  mniip             |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.4.3
  checker)                           |
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8423 #15546      |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mniip):

 Correction: an `apart` constraint might sometimes not get discharged if
 the same tyvar occurs multiple times, e.g:

 {{{
 type family Eq a b where
    Eq a a = True
    Eq a b = False
 }}}

 {{{
 TF a a ~ True
 apart(<c, c>, <a, b>) => TF a b ~ False
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15557#comment:4>
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